31 



has been so drastically altered that the study of organic geochemistry 

 of ancient rocks is reduced to the study of plain graphite. There is a 

 continuum between the extremes of "DNA" and "graphite." Sedi- 

 mentary organic materials must inevitably move along this contin- 

 uum as time progresses. As we move back more than 2.5 b.y. before 

 the present into the Archean, the question arises as to what we will 

 find first: The origin of life? Total degradation of all organic material 

 to uninformative graphite? Or the end of the record? For at least 

 15 years, every generally accepted minimum date for the appearance 

 of life on Earth has been based on morphological evidence alone 

 (stromatolites, micro fossils), i.e., visible or microscopic traces, not 

 molecular ones. Chemical analyses have been viewed as suggestive, 

 but not compelling. The approach suggested here acknowledges the 

 preeminence of such morphological evidence when it comes to a yes 

 or no question about the existence of life, but then works toward 

 more insight. Specifically, if micropaleontologists can show where 

 life existed, then perhaps organic geochemists can further support 

 that interpretation and can determine what kind of life it might have 

 been. 



The most important advance in organic geochemistry during the 

 past 10 years has been the emergence of a coherent view of the 

 nature and role of the substance called kerogen. This material is a 

 blackish and insoluble macromolecular complex dispersed in sedi- 

 mentary rocks and comprising the great majority of organic matter in 

 sediments. It has effectively resisted many efforts at detailed macro- 

 molecular structural analysis, presumably because it lacks the regular 

 structure like coal or the asphaltene particles in petroleum. But it is 

 now recognized as a product of geochemical reactions occurring over 

 a very long period of time. "Protokerogen," an organic substance 

 composed mainly of cellular degradation products, is formed today 

 by living microbial communities in modern sediments. It is added to 

 and modified by a series of reactions grading from the microbiologi- 

 cal through the high-temperature geochemical. The present level of 

 understanding is not complete and does not fully extend into the 

 Precambrian, but it raises questions regarding chemical fossils and 

 points the way to useful investigations of kerogens. 



The times of the origin of photosynthesis, of respiration, of 

 other biochemical processes of great ecological significance are not 

 now known. Morphological evidence alone is unlikely to be decisive 



